Process for producing an electrode pitch which can be easily graphitized

ABSTRACT

DISSOLVING A COAL TAR PITCH IN AN AROMATIC RICH OIL FRACTION CONTAINING NAPHTHALENE AND ITS HOMOLOGS AS ITS MAIN INGREDIENTS AND HAVING A BOILING RNAGE OF 200 TO 300*C., IN A WEIGHT RATIO PITCH TO OIL FRACTION OF 1:1 TO 1:4 TO. PRODUCE A SUSPENSION; SEPARATING THE SOLID CONTENT FROM THE SUSPENSION; DISTILLING OFF SAID OIL FRACTION FROM THE FILTRATE AT SUCH A SUMP TEMPERATURE AND WITH SUCH TIME OF STAY FOR THE PITCH FORMING THE SUMP PRODUCT OF THE DISTILLATION COLUMN THAT THE CONTENT OF B-RESINS IS INCREASED WITHOUT FORMING OF ADDITIONAL A-RESINS.

United States Patent 3,761,387 PROCESS FOR PRODUCING AN ELECTRODE PITCHWHICH CAN BE EASILY GRAPHITIZED Otto Wegener, Mulheim-Speldorf, HeinrichLouis, Duisburg-Hamborn, and Rudolf Oberkobusch and Gerd Collin,Duisburg-Meiderich, Germany, assignors to RutgerswerkeAktiengesellschaft, Frankfurt am Main, Germany No Drawing. Filed Nov.17, 1971, Ser. No. 199,774 Claims priority, application Germany, Nov.30, 1970, P 20 58 751.3 Int. Cl. Cc 3/02 U.S. Cl. 208-45 7 ClaimsABSTRACT OF THE DISCLOSURE Dissolving a coal tar pitch in an aromaticrich oil fraction containing naphthalene and its homologs as its mainingredients and having a boiling range of 200 to 300 C., in a weightratio pitch to oil fraction of 1:1 to 1:4 to produce a suspension;separating the solid content from the suspension; distilling 01f saidoil fraction from the filtrate at such a sump temperature and with suchtime of stay for the pitch forming the sump product of the distillationcolumn that the content of fi-resins is increased without forming ofadditional a-resins.

BACKGROUND OF THE INVENTION Processes have been known to produceanisotropic,

easily graphitizable cokes which can be used for the production ofgraphite electrodes. However, according to the present state of the art,binding agents which are suitable for these cokes have not been known;following the production methods of easily graphitizable cokes, it couldbe concluded that, fundamentally, coal tar pitches having a low contentof highest molecular, sootlike substances which are insoluble inquinoline and also insoluble in anthracene-oil, the so-called oz-resinscan be easily graphitized. However, pitches of this type yield,according to the state of the art, in mixture with easily graphitizablecokes, graphite electrodes of low mechanical strength and highconsumption. For this reason, the specifications of the graphiteelectrode producers require in the pitch used as binding agent, normallya relatively high minimum-content of or-resins. These or-resins causenamely during coking and graphitizing, the formation of a very strongframework of carbon and this means electrodes with satisfactorymechanical strength and low consumption.

The object of the present invention is to provide a process forproducing a pitch for electrodes which can be easily graphitized, aswell as yield graphite electrodes having satisfactory mechanicalstrength.

SUMMARY OF THE INVENTION The solution of this problem is based in parton the perception that pitches having a high amount of a constituent ofp-resins (defined as substances which are insoluble in benzene but aresoluble in anthracene oil) form a strong framework of carbon duringcoking and-in addition to this-the B-resins-like the rat-resins, yield ahigh share to the coking residue of the pitch. According to thisperception it had to be the aim to remove the oc-l'BSil'lS from a pitchas far as possible quantitatively, and simultaneously to increase theamount of fi-resins, if possible.

By dissolving the pitch in quinoline or anthracene-oil, and filteringoff the insoluble ingredients, the rot-resins will of course bequantitatively removed, but the amount of B-resins will not beincreased. Filtration of a pitch suspension in these solvents is ratherdiflicult due to the high viscosity of the solvents. This is trueparticularly in the ice case of quinoline. Practically the solvents mustbe used in a large excess, in order to guarantee a technically goodseparation of the soluble and insoluble material at all. In additionanthracene-oil has the disadvantage that its boiling range is extremelyclose to that of the pitch so that the exact separation of the solventfrom the pitch solution is difficult; on the one hand, the solventbecomes pitchlike to a certain extent and on the other hand certainparts of it distill already off so that the pitch characteristics havechanged after distilling off of the anthracene-oil in a uncontrolledmanner.

It has now been found advantageous to use instead of quinoline oranthracene-oil as solvents for the pitch an aromatic-rich oil fractionwith a boiling range of 200 to 300 C., preferably 220 to 250 C.containing naphthalene and its homologs as its main ingredients forseparating the u-resins. In view of their low viscosity and goodcoagulating action, these oil fractions may be used in comparatively lowamounts, generally in a small excess and preferably in a ratio(pitchzoil fraction) of from 1:1 to 1:4, more preferably about 1:2. Thesuspension can easily be filtered and about of the undesired a-resinsare separated off by the filtration. Particularly preferred oilfractions are those obtained from the residual oils of benzine pyrolysisto ethylene; such oil fractions have a lower viscosity thancorresponding fractions from coal tar.

It has been futher found when using coal tar pitches produced bycontinuous flash distillation, that the desired fl-resins can beincreased still further without forming of any additional a-resins if,during distilling oif again the solvent from the filtrate of the pitchsuspension such a pressure is maintained that the pitch forming the sumpproduct of the distillation column is maintained for a controlled periodof time at a controlled sump temperature. The two controlled values--thesump temperature and the dwell timevary somewhat for pitches ofdifferent sources but they can be easily determined by simple laboratorytests. The sump temperature, therefore, must be high enough thattit-resins may be formed but it must not be so high that additionalu-resins will be formed.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT OF THE INVENTION Thus thesubject matter of this invention is a process for producing an electrodepitch which can be easily graphitized and is characterized by thefollowing steps: A coal tar pitch is dissolved in an aromatic rich oilfraction of a boiling range of 200 to 300 C., preferably 220 to 250 C.which contain as main ingredients naphthalene and its homologs toproduce a suspension, the weight ratio of pitch to said oil fractionbeing from 1:1 to 1:4 preferably about 1:2; the resulting suspension isfiltered in the heat or separated; theadded oil fraction is distilledoff again at such a sump temperature and with such a time of stay forthe pitch forming the sump product of the distillation column that theB-resin content in said pitch is increased without forming additionala-resins.

When the coal tar pitch used is a pitch obtained by continuous flashdistillation of coking tars from the Ruhr-area the sump temperature atwhich the formation of [Si-resins occurs is 300 to 400 C. preferably 320to 360 C., the time of stay is 0.25 to 5 hours preferably 1 to 2 hours.By the process of this invention the B-resins of the charged pitch theportion of which lies generally between 15 and 20% may be increased by25-50% or more to produce an easily graphitizable electrode pitch havinga fl-resin content of more than 24%, e.g. about 26%. The coking residuesof the obtained electrode pitches are higher than 50% (according toConradson). On coking a firm carbon framework is obtained. Graphiteelectrodes manufactured from these pitches have in addition to goodelectrical and thermal properties a high mechanical strength.

The present process can be applied also to thermally after-treated coaltar pitches which are used as electrode pitches for non-graphitizedcarbon electrodes e.g. for aluminiu m smelting. When these pitches aretreated according to the process of invention also up to 90% of theundesired a-resins are removed. However the additional formation offl-resins takes place in small amount only since in the case of thesethermally treated electrode pitches the condensation process has alreadypartially taken place. However as these pitches have before beingsubjected to the process of invention, already got a fi-resins contentof more than 20% it is easy to obtain at the end electrode pitchescontaining more than 24% of Q-resins. It is true, of course, that asmall decrease of the coking residue occurs.

The invention is further explained by the following nonlimitingexamples:

Example I 5000 parts by weight coal tar pitch obtained by continuousflash distillation, were dissolved in 10,000 parts by weight of aresidual oil fraction, rich in aromatics of the benzine pyrolyse toethylene and the suspension thus obtained at 80 C. to 100 C. sucked offover a vacuum filter. The coal tar pitch had the followingcharacteristics:

Softening point Kramer-Sarnow C '70 Benzol insoluble percent 27.3Anthracene oil insoluble do 7.4 fl-resins do 19.9 Coking residue(Conradson) do 60.0

The oil fraction rich in aromatic had the following properties:

Density at 20 C. 1.008 Analysis by fractional distillation:

Start of boiling: C 226 10% C 228 50% C..- 234 90% C 247Gaschromatogramm: Naphthalene percent 31 Methylnaphthalene do 41Dimethylennaphthalene do 13 Diphenyl do 5 Other aromatic hydrocarbons doIn the filtration step 1050 parts by weight filter-residue and 13,950parts by weight filtrate were obtained. From the filtrate bydistillation under a pressure of 100 torr, 9,650 parts by weight of oilwere distilled off so, that in the sump of the distillation apparatus afinal temperature of 350 C. was obtained. This temperature was more than1 hour maintained in order to maintain a thermal aftercondensation. Thespecial electrode-pitch (4,250 parts by weight corresponding 85% yield)had the following properties:

Softening point Kramer-Sarnow C 86 Benzol-insoluble percent 25.4Anthracene oil-insoluble do 0.9 St-resins do 24.5 Coking residueaccording Conradson do 50.1

The comparison with the pitch used shows that due to the treatmentaccording to the invention not only almost 90% of theanthracene-insoluble material-which hinders the graphitizingis removed,but at the same time the 8- resins content being essential for thesolidity of the coke residue has been increased by almost 25%. The useof the special electrode-pitch made it possible to manufacture agraphite electrode having good electric and thermal properties and highmechanical strength.

Example II In a manner analogous to Example I, another coaltar pitchalso prepared by continuous flash distillation was dissolved in theratio 1:2 in the described oil fraction rich in aromatic, the resultingsuspension was filtered and the oil distilled, so that a sumptemperature of 320 C. was more than 2 hours maintained. The propertiesof the charged and of the pitch obtained in a yield of 83% were asfollows:

Special Pitch electrode charged pitch Softening point (K.-S.), C 75.5 90Benzol-insoluble, percent 25. 1 2G. 8 Anthraceneoil-insoluble, percent8. 1 0.8 B-Resins, percent 17.0 26. 0 Coking residue (Conradson), percen50. 8 53. 0

Thus, due to this treatment, the amount of the anthracene-insoluble hasbeen reduced by 90% and the amount of fi-resins more than 50% increased.

Example III Special Pitch electrode charged pitch Softening point(K.-S.), 0.... 83. 0 84. 5 Benzolinsoluble, percent 25. 4 25. 6Anthraceneoil-insoluble, percent. 9.1 1. 3 fi-Resins, percent 16. 3 24.3 Coking residue (Conradson), percent 53. 9 53. 2

This treatment reduced the content of substances insoluble inanthraceneoil by about 85% and increased the fi-resins content by 50%.

Example IV As described in Example I a normal electrode pitch obtainedby thermal aftertreatment of a coal tar pitch was dissolved in aaromatic rich oil-fraction in the weight ratio of 1:2 and the suspensionthus obtained was filtered and the oil distilled off in a manner thatthe sump temperature was maintained at 360 C. for 2 hours. Theproperties of the charged pitch and the yield of 85 produced itch were:

Charged Final pitch product Softening point (K.-S.), C 85 80 Benzolinsoluble, percent 32. 7 26. 4 Anthraceneoil-insoluble, percent 10. 5 1.2 fi-Resins, percent 22. 2 25. 2 Coking residue (Conradson), percent.54. 1 5 1 By the treatment of the already precondensed pitch, theanthraceneoil-insoluble material was by almost reduced, but the amountof [i-resins was increased only by 14%.

What is claimed is: 1. Process for producing an electrode binder pitchin high yield from coal tar pitch starting material, which binder pitchcan be easily graphitized comprising the steps of heating and dissolvingthe coal tar pitch in an aromatic solvent comprising as main ingredientsone or more n-methylnaphthalene materials where n is from Zero to 3 andhaving a boiling range of 200 to 300 C.,

the pitch to solvent weight ratio being at least 1:1

and no greater than 1:4,

to produce a suspension,

hot filtering the solid content from said suspension to produce afiltrate of reduced alpha-resin content compared to the starting pitch,

distilling olf said oil fraction from the filtrate at subatmosphericpressure in a distillation column,

and treating distillation sump product of the distillation column atsuch a sump temperature and with such a time of stay that the amount ofbeta-resins in the sump product is increased without formation ofadditional alpha-resins therein.

2. Process according to claim in which said aromatic rich oil fractionhas a boiling range of 220 to 250 C.

3. Process according to claim 1 in which the said coal tar pitch isdissolved in the said aromatic rich oil fraction in a weight ratio of1:2.

4. Process in accordance with claim 3 wherein the oil comprises 31%naphthalene, 41% methylnaphthalene and 13% dimethylnaphthalene,

the filtration is carried out at 80-110 C.,

the distillation is carried out at 0.13 atmospheres and reaches a finalsump temperature of 320-360 C., and

wherein the final sump temperature is maintained for 1-2 hours.

5. Process in accordance with claim 4 wherein the final sump temperatureis 350 C. and is held for one hour.

6. Process according to claim 1 in which coal tar pitch of the cokingtars from the Ruhr-area are used and the sump temperature is maintainedfrom 300-400 C., with a time of stay for the pitch of 0.25 to 5 hours.

7. The process of claim 6 wherein the stay time is 1-2 hours and thesump temperature is maintained from 320- 60 C.

References Cited UNITED STATES PATENTS 3,558,468 1/1971 Wise 208-81,722,872 7/ 1929 Weindel 20845 3,490,586 1/1970 Jaisle 208-45 3,147,2059/1964 Ohsol 208-45 FOREIGN PATENTS 1,080,866 8/ 1967 Great Britain20844 DELBERT E. GANTZ, Primary Examiner V. OKEEFE, Assistant Examiner

